1. Field of the Invention
The present invention relates to a differential amplifier that outputs a signal corresponding to a level difference between two input signals.
2. Description of the Related Art
Differential amplifiers of CMOS (Complementary Metal Oxide Semiconductor) structure having a current mirror circuit are known as an example of the differential amplifier described above. A differential amplifier of this type is for example shown in FIG. 2 of Japanese Patent Kokai No. H04-192619 (Patent Literature 1). In such a differential amplifier, analog input signals S1 and S2 are respectively inputted to the gate of each of n-channel MOS transistors 1 and 2. Consequently, a current corresponding to the input signal S1 flows into the re-channel MOS transistor 1 via a p-channel MOS transistor 3 that constitutes a current mirror circuit. At the same time, a current corresponding to the input signal S2 flows into the n-channel MOS transistor 2 via a p-channel MOS transistor 4 that constitutes the current mirror circuit.
In this case, a differential output signal corresponding to a level difference between the input signals S1 and S2 is obtained at a connection point between the n-channel MOS transistor 2 and the p-channel MOS transistor 3.
With respect to a differential amplifier constituting an operational amplifier, it is desirable that, immediately before the supply of the input signals to be differentially amplified, the input and output voltages are stabilized in the form of self-bias voltages of the operational amplifier. This is beneficial for obtaining an accurate output result.
However, when a large amplitude analog signal is inputted or when the sampling intervals are long in the case that a high-gain circuit is employed as the current mirror circuit of the differential amplifier, the output of the operational amplifier can be saturated and the MOS transistor constituting the current mirror circuit can operate in a linear region. As a result, when the differential amplifier is rendered to operate in such a state, a relatively long time is necessary to shift the operation region of the MOS transistor from the linear region to a saturation region even if the input and output terminals of the differential amplifier are short-circuited, and consequently a problem of a reduced processing speed will result.